Building a brick wall is a little complex. You will have to dig down and create footings even with a small wall otherwise it may fall down. However, as Daisy has suggested, you could buy or construct yourself, a seat around the tree like below. You can also buy metal ones.

Try some shade and drought tolerant plants if you want to grow owt underneath it.

The ring of bricks above give a fair idea of the look. irregular stone blocks might look more natural.

A narrow level trench (footings) will indeed be best to do it properly. Then some aggregate, concrete, and blobs of mortar (concrete with more sand added) between each block as you build the levels up. There will be countless vids online showing 'how-to'.

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